This proposal is designed to investigate certain molecualr interactions at the cell surface of preimplantation mouse embryos which regulate compaction of the morula and its subsequent transition to the blastocyst stage. For this purpose, I intend to study embryos homozygous for the earliest acting lethal mutation of the T/t-complex, t12, since such embryos apparently fail to make the appropriate cellular connections and, therefore, do not undergo compaction of blastogenesis. Evidence has accumulated which suggests that t12/t12 embryos are characterized by abnormalities in surface galactosylation and by decreased expression of the onco-foetal antigen, F9. Conversely, abnormalities in surface galactosylation of wild type embryos induced by treatment with UDP-glactose or Lambda-lactalbumin, transforms them into t12/t12 phenocopies; such embryos can be protected form the teratogenic effect of these compounds by pretreatment with an antiserum directed against the F9 antigen. The simplest hypothesis to explain these observations is theat the t12 mutation, UDP-galactose, and Lambda-lactalbumin all act by altering the activity of a cell surface gaolactosyltransferase. One of the substrates for this transferase appears to be the F9 antigen which must be present in its appropriate ("normal") configuration in order for the embryo to compact the proceed through subsequent differentiative events. This proposal outlines a series of experiments designed to evaluate this hypothesis. Cell surfaces of both normal and mutant embryos will be labeled by various techniques and analyzed by gel electrophoresis and column chromatography in order to identify the relevant molecules. The N-acetylglucosamine: galactosyltransferase will be isolated from both wild type and mutant cells either by immunoprecipitation with a specific antibody against this enzyme or by affinity chromatography on Lambda-lactalbumin-Sepharose columns. In addition, the expression of the F9 antigen on these embryos will be examined serologically. Besides providing much needed information on specific cell surface interactions during development, these studies will facilitate the rescue and propagation of t-mutant embryos thereby providing material for in depth genetic and biochemical analyses of T/t-complex products.